DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS
TABLE 1 List of Ingredients Used in the Composite Propellants Tested Ingredient BAMO-AMMO: BAMO-AMMO polymer GAP plasticizer AP (200 ␮m) AP (20 ␮m) Al (5 ␮m) Desmodur N-100 Triphenyl tin chloride Weight % 7. However. 2. reliable predictions cannot be made without extracting reliable kinetic information from slow heating rate data. HTPB-DOA-AP ϳ0.9 mg. The chemical composition of the propellants is shown in Table 1. BAMO-AMMO ϳ1. This study focuses on obtaining reliable kinetic information on the thermal decomposition PBAN and three different solid propellants (Table 1).25
can be used to predict decompositions under conditions of combustion [6].9 mg. The instrument was programmed for heating at constant rates from room temperature to 590°C.94 5.00 0. This means that the effective activation energy determined from TGA experiments will also be a function of these two variables.60 12. Another major ﬂaw of this approach is that usually the model-ﬁtting methods produce a single pair of Arrhenius parameters for the whole process. To reduce thermal gradients and self-heating the experiments were performed on small samples (PBAN ϳ2.0 mg). The model-ﬁtting methods are simply incapable of accounting for this degree of complexity.01 Ingredient PBAN-AP: PBAN AP (2 ␮m) HTPB-DOA-AP: HTPB (R45M) DOA AP (2 ␮m)
175
Weight % 29. a commonplace approach is force ﬁtting of experimental data to assumed reaction models. whose substitution into Eq. Samples of the propellants were placed in open aluminum pans and heated in a ﬂowing atmosphere of nitrogen (100 ml minϪ1). PBAN-AP.40 18.81 70.42 0. Kinetic Analysis of TGA Data The kinetics of heterogeneous decompositions of solids are customarily described by the basic kinetic equation d␣ ϭ k͑T͒ f͑␣͒ dt (1)
f ( ␣ ) is the reaction model. the thermal decompositions of solid materials are known to involve multiple steps that are likely to have different activation energies. HTPB-DOAAP.58 11. one has to separate the temperature and conversion dependencies of the reaction rate.76 70. Here. and BAMO-AMMO propellants were kindly supplied by the Thiokol Corp. and
where E is the activation energy and A is the preexponential factor. The thermogravimetric analysis (TGA) experiments were carried out using a Rheometrics 1000Mϩ thermobalance. which describes the dependence of the reaction rate on the extent of reaction. When applied to decomposition data obtained under nonisothermal conditions.24
23. The value of ␣ is experimentally derived from the global mass loss in TGA experiments. t is time.99 49. However. In most cases the temperature dependence of k ( T ) can be satisfactorily described by the Arrhenius equation. Experimental Section Samples of PBAN. the contributions of these steps into the overall decomposition rate measured by TGA should vary with both T and ␣.8 mg. PBAN-AP ϳ0. k ( T ) is the rate constant. The above-mentioned problems can be avoided by using model-free isoconversional methods
. Then. the model-ﬁtting method gives highly uncertain values of the Arrhenius parameters [7]. 1 yields ϪE d␣ ϭ A exp f͑␣͒ dt RT
ͩ ͪ
(2)
where ␣ represents the extent of reaction (0 Յ ␣ Յ 1). To evaluate E and A in Eq.

under a linear heating program distorted by self-heating) [12]. TGA curve for decomposition of PBAN at heating rate of 0. T j ͑ t ␣͔͒
(3)
Henceforth. this is the ﬁrst work to apply such a method to thermal decomposition of composite rocket propellants. SELL ET AL. These methods allow the activation energy to be evaluated without making any assumptions about the reaction model. 1. and 9. The minimization procedure is repeated for each value of ␣ to ﬁnd the dependence of the activation energy on the extent of conversion. The ﬁrst step covers a
temperature region from ϳ110 to ϳ220 °C and involves a mass loss of ϳ8%.
iϭ1 j i
͸͸
n n
J ͓ E ␣. T i ( t ). After decomposition we found a small amount of char that accounted for 1% of the initial mass. There is also a minor step of the mass loss (ϳ2%) with the maximum rate at ϳ 100°C.176 [8 –10]. 2. Figure 2 compares the E ␣ -dependence against the experimentally observed mass loss curve. 7. The thermal decomposition noticeably accelerates in the third step. The steps are strongly overlapped. Additionally. 1. the isoconversional methods evaluate the effective activation energy as a function of the extent of conversion. the activation energy is determined at any particular value of ␣ by ﬁnding the value of E ␣ that minimizes the function ⌽͑ E ␣͒ ϭ
T.g. the subscript ␣ denotes the values related to a given extent of conversion. RESULTS PBAN Polymer Binder Thermal decomposition experiments on the neat PBAN binder were performed at constant heating rates of 0. T ( t ).9. T i ͑ t ␣͔͒ J ͓ E ␣.3. This step is hardly noticeable at faster heating rates. for a set of n experiments carried out at different heating programs.1 °C minϪ1. To our knowledge.6. in which the sample loses ϳ75% of mass in a temperature region of ϳ380 –500 °C.
Fig. The representative TGA curves are shown in Fig. According to this method. the integral J ͓ E ␣. The three decomposition steps are associated with variations in the E ␣ -dependence. Vyazovkin developed an advanced isoconversional method [12]. TGA curves for decomposition of PBAN.9°C minϪ1 (upper trace) compared to the dependence of the activation energy on the extent of decomposition conversion (lower trace). The second step demonstrates a mass loss of 15% in the temperature region of ϳ220 –380 °C. which allows one to explore multistep kinetics [11]. 2.
Fig. T i ͑ t ␣͔͒ ϵ
͵ ͫ ͬ
t␣
exp
0
ϪE ␣ dt RT i͑ t ͒
(4)
is evaluated numerically for a set of experimentally recorded heating program. An advantage of the advanced isoconversional method is that it can be applied to study the kinetics under arbitrary temperature programs (e. which is based on the assumption that the reaction model is independent of the heating program. 4. 3.7.. Open symbols represent literature data [13]. In Eq.8. 1. The PBAN polymer shows three major steps of mass loss.
.

3) thermal runaway at the heating rate of 4. which do not undergo thermal runaway. 4. TGA curves for decomposition of PBAN-AP composite propellant.9.3.25– 0.3. It precedes the ﬁrst major step that shows a fast mass loss of around 70% in the temperature region of 100 –350°C. 0. 3.7. TGA curves for decomposition of HTPB-DOA-AP composite propellant.0 °C minϪ1.5. At slower heating rates.2.3. At slower heating rates the TGA curves show two major steps of mass loss.9. 2.5°C minϪ1) decompositions resulted in formation of a char residue that accounted for ϳ15% of the initial mass. In the third step (␣ ϭ 0. The second step demonstrates a fast mass loss of ϳ70% at temperatures between 230 and 380
Fig. 3. 8.5.5. The slow second step (␣ Ͼ 0. During the ﬁrst step the propellant loses 8% of its mass in the temperature range of 100 –230°C. The slower heating rate (Ͻ4. 4.2.9.08) is characterized by a small activation energy of 50 –70 kJ molϪ1.1.
activation energy. 1.25) is characterized by an increase in E ␣ from ϳ100 to 260 Ϯ 40 kJ molϪ1. The TGA experiments showed (Fig. and 9.5°C minϪ1 and faster.8) shows a rise in the activation energy from ϳ20 to 180 kJ molϪ1.6.
. The major decomposition step (␣ Ͻ 0. 6.08 – 0. Thermal runaway was observed at the heating rates faster than 3 °C minϪ1 (Fig. 4. 4. PBAN-AP Composite Propellant The composite PBAN-AP propellant was studied at constant heating rates of 0. 4.5. we can also see a minor mass loss (ϳ2%) with maximum rate at ϳ 100°C.8.8. HTPB-DOA-AP Composite Propellant The thermal decomposition of HTPB-DOA-AP composite propellant was examined at constant heating rates of 0. TGA curve for decomposition of PBAN-AP at heating rate of 0. and 9. 1.0 °C minϪ1. the TGA curves show 3 major steps of the mass loss. 2. The results are presented in Fig. 5). 5. 3.
The ﬁrst step (␣ Ͻ 0. 2. 2. As in the case of neat PBAN.5.99) the activation energy rapidly decreases from 260 Ϯ 40 kJ molϪ1 to a practically constant value of 200 Ϯ 30 kJ molϪ1. The average activation energy for this step is around 40 kJ molϪ1.DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS
177
Fig. The second step (␣ ϭ 0. The E ␣ -dependence was evaluated for the slow heating rate decompositions. 3. The second step occurs above 350°C and demonstrates a slow mass loss of ϳ15%.8) starts with an abrupt drop in the
Fig.9°C minϪ1 (upper trace) compared to the dependence of the activation energy on extent of decomposition (lower trace).5. 0. 7.

TGA curve for decomposition of BAMO-AMMO composite propellant at heating rate of 7. TGA curves for decomposition of BAMO-AMMO composite propellant. TGA curve for decomposition of HTPB-DOA-APpropellant at heating rate of 0.9). 7. The third step ( T Ͼ 320°C) is a slow decomposition
accompanied by a small mass loss of 4%.0 °C minϪ1.
Fig. When the temperature reached the ﬁnal value of 590°C.5.178
T. and 9.2.
. The results are presented in Fig.2°C minϪ1 (upper trace) compared to the dependence of activation energy on the extent of decomposition (lower trace). 8. 7. 6. the propellant loses more than 50% of its mass. The third step exhibits an abrupt change of the activation energy to very low values that could not be reliably evaluated. which shows an increase in E ␣ from ϳ100 to 230 kJ molϪ1. The results are shown in Fig. The ﬁrst step of decomposition (␣ Ͻ 0. By 590°C about 4% of the propellant mass remained.9°C minϪ1 (upper trace) compared to the dependence of the activation energy on extent of decomposition (lower trace). BAMO-AMMO Composite Propellant The BAMO-AMMO composite propellant was decomposed at constant heating rates of 0. SELL ET AL. 6.9. The activation energy suddenly drops at the beginning of the second step (0. As in previous cases.9) is practically constant (120 Ϯ 20 kJ molϪ1) at ␣ Ͼ 0. The thermal decomposition of this propellant is characterized by the E ␣ -dependence shown in Fig. The ﬁrst stage of decomposition shows an increase of the activation energy from 100 to 160 kJ molϪ1 at ␣ ϭ 0. At the beginning of the second step the activation energy suddenly drops to 80 kJ molϪ1.08 Ͻ ␣ Ͻ 0. The transition to the third step of decomposition is characterized by some increase in activation energy that is followed by the rapid drop at ␣ ϭ
Fig. 7.2– 0. The third step (T Ͼ 380°C) shows a very slow decomposition accompanied by 5% of loss in mass. 8. The TGA curves show three major steps of the mass loss.
Fig. the ﬁrst step is preceded by a minor mass loss with the maximum rate around 100°C.3. During the ﬁrst step the propellant loses ϳ15% of its mass in the temperature range 150 –250 °C.3. 2. the samples had less than 10% of the initial mass remaining. 4. The activation energy for the second step (␣ ϭ 0.
°C.2. The E ␣ -dependence was determined for decompositions conducted at heating rates slower than 3°C minϪ1. In the second step ( T ϭ 250–320 °C).08) exhibits an activation energy of about 100 kJ molϪ1.

Therefore
179
the overall kinetics of PBAN decomposition appears to be determined by two overlapping processes with the respective activation energies of ϳ100 and 200 kJ molϪ1. When heated at 5°C minϪ1. Cohen et al. This also applies to the data of Rao and Radhakrishnan [14] who measured the total amount of C2 hydrocarbons released from PBAN. We feel that generally they represent a computational artifact. they obtained the activation energy of 52 kJ molϪ1.25– 0. The use of the Ozawa method [9] resulted in a E ␣ -dependence (Fig. neat
. The third step has an activation energy of ϳ20 kJ molϪ1. we assume that this initial step is associated with vaporization moisture from the above systems. Based on this fact and on the proximity of the maximum rate of mass loss to 100°C. This step may result from vaporization of a residual organic solvent or some other species of low molecular mass. It suggests that this step is likely to involve two decomposition pathways [11]. As was noticed.DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS 0. which was pyrolyzed at different temperatures. The somewhat higher values of the activation energy may be explained by the effect of curing that is likely to result in formation of a more stable polymer network. the evaluated E ␣ -dependencies sometimes show abrupt drops in the activation energy. Overall. With increasing the temperature. Cohen et al. The second decomposition step (␣ ϭ 0. which uses an oversimpliﬁed approximation for the temperature integral (4) that holds only for ideally linear heating program and cannot account for appreciable self-heating. By plotting the logarithm of the mass loss ﬂux against the reciprocal surface temperature. It should be kept in mind that the PBAN-AP propellant is mostly (70%) composed of AP.08) shows the activation energy (ϳ50 – 60 kJ molϪ1) that is uncharacteristically small for the thermal decompositions. However. The values of the activation energy related to that step are too small (20 –50 kJ molϪ1) for a chemical reaction (bond breaking). Under assumption of ﬁrst-order kinetics.08 – 0. they found the activation energy of 70 kJ molϪ1. This minor step is absent in the thermal decomposition of HTPBDOA-AP propellant. it may also be due to the limitations of Ozawa’s method. The major mass loss step starts at ϳ200°C and accounts for about 80% of the mass loss. Rao and Radhakrishnan [13] studied the thermal decomposition of cured PBAN by using TGA at heating rates between 2 and 50 °C minϪ1. [3] used Xe lamp radiation to surface pyrolyze PBAN. we cannot meaningfully compare our results with those obtained by Cohen et al. PBAN-AP Composite Propellant The thermal decomposition of the PBAN-AP propellant does not show the mass loss between 110 to 220 °C that we observed for the neat PBAN binder. This is a crucial factor because according to the present results the activation energy varies with the extent of PBAN degradation. 2) that has a similar shape as that obtained by us. Besides.95. [3] did not report the extent of pyrolysis reached in their experiments. The validity of this value is obviously limited by an arbitrary assumption of zero-order kinetics. [3]. The low temperature sensitivity of the reaction rate manifests itself in a small value of the activation energy. The value is too small to represent a chemically controlled decomposition. it is reasonable to assume that the third and second steps may involve a common reaction pathway. DISCUSSION The neat PBAN as well as PBAN-AP and BAMO-AMMO propellants showed a minor initial mass loss (ϳ2%). Note that these drops occur at the point of transition from one mass loss step to another.25) demonstrates an increasing E ␣ -dependence. The reaction rate at this point markedly decreases and does not vary much with temperature. PBAN Polymer Binder The ﬁrst mass loss step (␣ Ͻ 0.99) is strongly overlapped with the second one and that the activation energy for the third step is 200 Ϯ 30 kJ molϪ1. Given the fact that the third decomposition step (␣ ϭ 0. the pathway having a smaller activation energy (ϳ100 kJ molϪ1) is taken over by the pathway that has a greater activation energy (ϳ200 kJ molϪ1).

At this heating rate.180 AP completely decomposes in the temperature region of ϳ240 –350°C [7]. However. Based on these facts we may conclude that this step involves the thermal decomposition of all AP as well as of ϳ10% of HTPB. they ﬁtted data to the reaction-order model. that the mass loss is practically equal to the content of the DOA plasticizer in the propellant. which are decomposition of individual PBAN and reaction of PBAN with decomposition products of AP. by 350°C neat PBAN loses only about 10% of its mass (Fig. the maximum value of the activation energy for the propellant decomposition is lower (180 kJ molϪ1) than that for the decomposition of neat PBAN (260 kJ molϪ1). During this step the propellant loses ϳ80% of its mass. the overall kinetics of the ﬁrst and major decomposition step can be reduced to two parallel pathways. However. 2). however. On the other hand. According to Vyazovkin and Wight [7] the thermal decomposition of AP is characterized by a complex E ␣ dependence for which the maximum value of the activation energy does not exceed 120 kJ molϪ1. These values are noticeably smaller than those found by us. we may expect that the overall kinetics of this step is determined by decomposition of HTPB or/and by its reaction with the products of AP decomposition. Note
T. The corresponding activation energy is about 100 kJ molϪ1. neat PBAN decomposes at the temperatures between 200 and 480 °C. It does not seem to be unreasonable to assume that the ﬁrst step relates to the vaporization or decomposition of the plasticizer. Assuming zero-order kinetics. rule out that the HTPB may also contribute to this ﬁrst step. We cannot. the maximum activation energy of the PBAN propellant decomposition is likely to characterize the reaction between PBAN and decomposition products of AP. they found the activation energy of 71 kJ molϪ1. Cohen et al. The reported values [15] of the activation energy correspond to a reaction of zero order and randomly vary from 80 to 150 kJ molϪ1 depending on the heating rate as well as on the computational method used. SELL ET AL. The activation energy for this step increases from ϳ100 to ϳ280 kJ molϪ1. We may therefore expect that the ﬁrst step of the propellant decomposition represents the decomposition of all AP and about onethird of PBAN. Because the average activation energy for decomposition of the propellant is signiﬁcantly greater than that for decomposition of pure AP [7]. 4) is suggestive of a complex reaction mechanism.25 (Fig. Taking into account that the PBAN propellant begins to decompose at the temperature that is ϳ40°C lower than the decomposition temperature of neat AP. Rao and Radhakrishnan [14] measured the total amount of C2 hydrocarbons that was released from pyrolyzed HTPB at different temperatures. we should ascribe the initial smaller activation energy (ϳ80 kJ molϪ1 at 200°C) to the thermal decomposition of PBAN. the maximum activation energy for the propellant decomposition is markedly higher than that for decomposition of neat AP. [3] studied the kinetics of the surface pyrolysis of HTPB.8) suggest that further decomposition may be limited by diffusion [11]. pure HTPB was not available for this study to clarify this situation. However. The low rate and small activation energy (we were unable to reliably determine its value) of the third step suggest that the decomposition of the residual polymer is likely to be diffusion-controlled. the reliability of the reported values [15] is rather questionable because of the ﬂawed nature of the modelﬁtting methods [7]. The slow rate and low activation energy for the second decomposition step (␣ Ͼ 0.9) practically coincides with the temperature region of AP decomposition [7]. The E ␣ -dependence determined for the ﬁrst and major step of the propellant decomposition (Fig. Ninan and Krishnan [15] used TGA to study the kinetics of thermal decomposition of HTPB binder at heating rates of 1–100 °C minϪ1. Therefore. To evaluate Arrhenius parameters. Unfortunately.1 Ͻ ␣ Ͻ 0. The temperature region of the second and major mass loss step (0. By assuming ﬁrst-order kinetics for this process. HTPB-DOA-AP Composite Propellant The ﬁrst decomposition step (␣ Ͻ 0. Note that this dependence shows an increase in the activation energy that is similar to that observed for the PBAN decomposition at ␣ Ͻ 0. Therefore. 1).05) has the maximum rate at 150°C. they obtained the activation
.

Acta 215:315 (1993). Chem. W.. V. Appl. Chem. and Brill. K. 8). Phys. C.. 41:2251 (1990). J. 8) observed for the propellant decomposition is likely to represent the reaction of AP decomposition products with the polymer binder. including two polymers (BAMO-AMMO and GAP). J. and Wight. H. Pyrotechnics 20:32 (1995). The constancy of the effective activation energy suggests that the overall kinetics of this step is limited by a single reaction.. accepted 10 March 1999
. and Derr.. Thermochim. the second step occurs in the same temperature region as the temperature decomposition of neat AP [7]. T. Flynn. Cohen. J. I. M. This research is supported by the Ballistic Missile Defense Organization and the Ofﬁce of Naval Research under MURI Contract No. J.. T. Rao. we may conclude that the thermal decomposition of AP is accompanied by decomposition of the polymers. S.. Price. C 6:183–195 (1964 – 65). AIAA J. A. Polym. as well as by the reaction of the polymers with decomposition products of AP. J. 6. 8.. Soc. V. revised 25 February 1999. H.. Additionally. Int. L. Bull. Although the nature of the mass loss step is unclear. Kinet. Fleming. S. and curing agents.. Chem. B. S. Combust... I. A. 11. Therefore. S.. Arisawa. H. Vyazovkin. and Wight. A 101:7217 (1997). 12:212 (1974). S. CONCLUSIONS As a result of this study we found that the thermal decompositions of the PBAN binder and PBAN. Vyazovkin. REFERENCES
1. The propellant contains seven components.. A. Ozawa. A. Friedman. W. the E␣dependence (Fig. Nat. 28:95 (1996). BAMO-AMMO Composite Propellant Interpretation of the overall kinetic data for this propellant is extremely difﬁcult. Comput. H.. Rev. This step represents the major mass loss of ϳ70%. Ninan. 2.. Int.
7. S. T. T. Sci. 14. Chem. Bur. L.. and BAMO-AMMO propellants follow multistep kinetics. We have already questioned the validity of such values in our discussion of PBAN data. and Wall. Arisawa.. 17:407 (1998).S. E. Spacecraft Rockets 20:320 (1983). Flame 106:131 (1996). Sci.. For the thermal decomposition of neat AP the activation energy varies within the limits of 80 –120 kJ molϪ1 [7]. G.. Rao. Propellants. 12. HTPB. I. S. has been supported by a scholarship from Deutsche Akademischer Austauschdienst (DAAD). B. T. N. 15. 4. Vyazovkin. 13. N00014-951-1339.. C.. Standards 70A:487 (1966). R. the overall decomposition kinetics may be complicated by curing of the polymers.. Polym.. J. Res. 9. Comparison of the activation energies for this process with the activation energies for decomposition of individual AP or/and binder suggests that the overall kinetics of the major mass loss is determined by the reaction between binder and decomposition products of AP.. F. T. N. and Brill. S. Chem. and Radhakrishnan. Vyazovkin.DECOMPOSITION OF PBAN AND ROCKET PROPELLANTS energy of 53 kJ molϪ1.. M.95) has a low activation energy and reaction rate that are characteristic of diffusion control. The overall kinetics for this propellant may be determined by decomposition of individual polymers and AP. J. and Lesnikovich. Phys.
Received 5 August 1998. The major mass loss step for decomposition of propellants is the thermal decomposition of AP accompanied by partial degradation of the polymer binder. 18:393 (1997). Levchik. Vyazovkin. Vyazovkina. A. J. The use of the advanced isoconversional method allows one to
181
observe variation of the effective activation energy with the extent of decomposition. 10. 3. two sizes of AP. K. Combust. R. 5. R. and Radhakrishnan. Japan 38:1881 (1965). Flame 106:144 (1996)... The activation energy for the second step remains fairly constant at 120 Ϯ 20 kJ molϪ1 (Fig. R. Explosives. and Krishnan. Goryachko. Since this amount exceeds the overall amount of AP in the propellant. Spacecraft Rockets 19:92 (1982).. J.. The third decomposition step (␣ Ͼ 0. aluminum..